The present invention concerns the venting of gas from fuel tanks. In particular, it concerns venting fuel tanks of aircraft. Also particularly, but not exclusively, this invention concerns a tank vent device for venting gas from a tank. The invention also concerns a vent tank, a method of venting gas from a tank and a method of refuelling an aircraft including performing the method of venting gas.
It is important to vent a tank for two reasons. Firstly, a tank must be vented to enable the tank to be efficiently filled with fuel. If there is inadequate venting, the pressure build up in the tank is significant and can make it very difficult to put further fuel in the tank. Secondly, a pressure build up can result in a large positive pressure differential between the tank and the atmosphere. A negative pressure differential could also arise (depending on the atmospheric pressure). Any pressure differential puts undue strain on the tanks and can cause damage to them and their surrounding structure. This is especially important in relation to aircraft fuel tanks, as the fuel tanks are often located in the wings or tail and it is important to minimise and, preferably, entirely prevent, wing/tail damage.
Venting of aircraft tanks is normally performed by allowing gas to vent from the fuel tanks via a passive vent. The gas can be vented directly from the fuel tank itself or via a dedicated vent tank.
GB 2 321 639 A discloses a fuel vapour recovery system for an automotive vehicle. The system includes first and second vapour recovery canisters and a bypass flow element between the canisters. When the bypass flow element is open, vapour can pass directly through the bypass element to the second canister without first passing through the first canister. When the bypass flow element is closed, vapour must first pass through the first canister before reaching the second canister. The bypass flow element is opened when the pressure differential across the element is large enough to push a check valve open.
The vapour recovery system of GB 2 321 639 A can effectively contain the fuel vapour, that would otherwise be vented into the atmosphere as VOCs (volatile organic compounds) and cause air pollution. The fuel vapour can also be reused/recycled.
However, use of the vapour fuel recovery system of GB 2 321 639 A results in a vapour flow restriction (even when the bypass flow element is open). Therefore, the system causes pressure to build up in the fuel tank during refuel and therefore slows down the refuelling rate that can be achieved and puts undue strain on the tank. The system is also not suitable for use with jet fuel or for use on an aircraft.
U.S. Pat. No. 5,575,441 discloses a device for preventing fuel spillage and the venting of fuel vapour to the atmosphere. The device is attached to the outlet of a dump mast on a wing of a military aircraft during refuel. An adaptor assembly of the system is attached on one side to the outlet by a groove corresponding to the shape of the outlet walls. A standard fuel hose is attached to the opposite side of the adaptor assembly and feeds into a fuel container of a fuel truck. Suction is applied to the fuel hose in order to urge fuel and vapour out of the dump mast into the fuel container. However, the system disclosed contains no safety features that could prevent over or under pressure of the fuel tank or prevent the possibility of flame propagation via the fuel hose into the fuel tank. The system, however, does contain an alarm which can be used to show fuel flow problems or hose kinking. The alarm is controlled by sensors. Hence, electronics are used to control and provide warnings in relation to the vent system. This means that electrical power would be in close proximity to highly volatile fuel vapour, directly linked to the fuel tank. Failure of the safety system by an electrical short would give rise to a risk of fuel vapour ignition.
WO 2007/138366 discloses the use of a standard coupling (known as the API 1004 coupling) connected to an aircraft vent during refuel of the aircraft tanks. The coupling is connected back to a fuel tanker via a discharge line. The tanker collects the vapour and later transports it to a gas storage terminal. The gas can then be used for electricity production and supply. The coupling includes an “overpressure vent valve” which prevents the aircraft tank becoming overpressured by opening when the pressure in the aircraft tank increases too much. This coupling system is controlled solely based on the pressure in the aircraft tank. In other words, gas is extracted into the discharge line when the valve is open but not when it is closed. There is no independent control of the system. Furthermore, the system disclosed contains no safety features that could prevent the possibility of flame propagation via the discharge line into the aircraft tank.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved tank vent device, vent tank and method of venting gas from a tank.